Nature abounds with examples of how the properties of living systems are correlated to (determined by) their sophisticated, multiscale (hierarchical macro/micro/nano) structure, such as the self-cleaning effect of lotus (Nelumbo nucifera) leaves, the colors of butterfly wings and peacock feathers, the adhesive properties of gecko feet, the anisotropic wetting function of rice leaves, the anti-reflectivity of some insect wings and eyes, etc. Inspired by such living systems, a great number of studies have been conducted, which resulted in real-world technologies, e.g., coatings with self cleaning properties, photonic structures which can serve as optical waveguides and beam splitters, etc. The comprehensive reviews on bio-inspired smart materials and their applications can be found in NPL (Non-patent literature) Nos. 8-12, listed below, for example.
More interestingly, the nature is full of examples of living organisms exploiting the relationship between materials responsive properties and multiscale structure to respond efficiently to external stimuli. The multiscale MS system of living organisms—composed of a skeleton made of structured bones, muscles, tendons, ligaments, etc.,—provides shape, support, flexibility, stability and movement to the body, making it possible to lift large loads (NPL Nos. 13-16). The significance of the multiscale structure-property relationship in such organisms is that their responses to stimuli are considerably pronounced, however such relationship is not well understood (NPL Nos. 8-11).
In more specific context, Metal-Polymer nanocomposites are of particular interest for a variety of reasons, such as ease of processability, prospects for large-scale manufacturing, and considerably lower density than pure metals (NPL Nos. 33-38). Furthermore, they exhibit tunable optical and mechanical properties which can be realized by altering their geometry and composition (NPL Nos. 34-38). In particular, Au-PEI nanocomposites with size and shape dependent surface plasmon resonance (SPR) have been extensively investigated (NPL Nos. 39-42), for various applications ranging from biosensing (NPL No. 39) to gene expression (NPL No. 40). For example, a colorimetric assay method for the quantitative detection of heparin has been studied using Au-PEI nanoparticles (NPs) as an optical probe (NPL No. 39). The detection principle is based on a simple electrostatic interaction between the positively charged Au-PEI NPs and negatively charged heparin in solution that leads to aggregation of the Au-PEI NPs and hence a red-shift in the UV-Vis absorption signal. Au-PEI NPs have been also utilized as a potential non-viral gene carrier for intracellular siRNA delivery (NPL No. 40). The PEI polyelectrolyte acts as both the reductant and stabilizer on the formation of colloidally stable Au-PEI NPs, which binds siRNA electrostatically without showing any significant cytotoxicity (NPL No. 41). Recently, self-assembling 2D nanocomposities by PEI-stabilized AuNPs has gained increased attention in the field of nanofabrication for the development of multifunctional optical devices (NPL No. 42). Au-PEI NPs have been assembled at a solvent/water interface (e.g., toluene/water) into a 2D film with high surface area-to-volume ratio for plasmonic enhancement and surface-enhanced Raman scattering (SERS) (NPL No. 42).